EVOLUTION OF THE MODERN RAILWAY BRIDGE. 477 



the center of the river may be less than that required by naviga- 

 tion, so that, from what we now have, our only remedy would be 

 the use of the suspension bridge. 



In order to do away with this, and make a wider opening in 

 the center of the river, suppose we take away the center pier and 

 replace it by two piers directly under the king-posts of the truss 

 (Fig. 30). In this way we see we have left a large, clear span in 



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CANTILEVER. 



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 Fig. 30 



the center of the river, and we have in no way increased the 

 amount of materia^ necessary for the building of the bridge, and 

 the two spans that we are using are now balanced, each upon the 

 top of its respective pier. These two spans are fastened together 

 in the center, and the shore ends of both are anchored firmly, 

 in order to keep them from tipping up whenever a load comes 

 upon the river end. We thus see that we have doubled the clear 

 span in the center of the river, and we have what is called a can- 

 tilever bridge ; that is, a truss supported at one end, and extending 

 out over an opening, there being no support under the other end. 

 Now, suppose it was desired to make this center opening still 

 larger, we have simply to move the piers apart (Fig. 31). We 



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have our two cantilever spans. The shore end of each is firmly 

 anchored down, and the two other ends, A B, project simply into 

 space. If we build thus, the two ends, A B, are firmly fixed 

 and can not in any way yield to the load that may come upon 

 them. If we now construct an ordinary framed truss, of either 

 the Howe, Pratt, or any other type, and instead of putting this 

 truss upon two piers or abutments we simply hang it between the 

 two ends of the cantilever spans, A B, which are projecting over 

 the river, the weight of this truss will be sustained by the tie-rods 

 from the king-posts, or in the case of the cantilever that run over 

 the tower and are anchored down upon the other side. We thus 

 see that by increasing very slightly the amount of material used 



